Exhaust emission control device of internal combustion engine

ABSTRACT

An exhaust control valve ( 24 ) is arranged in an exhaust pipe ( 22 ) of an internal combustion engine. At the time of engine startup and warmup operation, the exhaust control valve ( 24 ) is substantially fully closed, the amount of injection of main fuel is increased from the optimum amount of injection at the time when the exhaust control valve is fully opened, auxiliary fuel is additionally injected during the expansion stroke, and thereby the unburned hydrocarbons exhausted into the atmosphere is greatly reduced at the time of engine startup and warmup operation.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification device ofan internal combustion engine.

BACKGROUND ART

In a diesel engine, at the time of low speed, low load operation of theengine, in particular at the time of warmup operation of the engine, thetemperature inside the combustion chamber becomes lower and as a resulta large amount of unburned hydrocarbons is generated. Therefore, knownin the art has been a diesel engine having an exhaust control valvearranged in an engine exhaust passage, closing the exhaust control valveand greatly increasing the amount of fuel injection at the time ofengine low speed, low load operation so as to raise the temperature inthe combustion chamber and cause the injected fuel to completely burn inthe combustion chamber and thereby keep down the amount of generation ofunburned hydrocarbons (see Japanese Unexamined Patent Publication(Kokai) No. 49-801414).

Further, when arranging an exhaust purification catalyst in an engineexhaust passage, if the temperature of the catalyst does not becomesufficiently high, a good exhaust purification action is not obtained bythe catalyst. Therefore, known in the art is an internal combustionengine which injects auxiliary fuel during the expansion stroke inaddition to the injection of the main fuel for generating the engineoutput and causes the auxiliary fuel to burn so as to cause thetemperature of the exhaust gas to rise and thereby cause thetemperature. of the catalyst to rise (see Japanese Unexamined PatentPublication (Kokai) No. 8-303290 and Japanese Unexamined PatentPublication (Kokai) No. 10-212995).

Further, known in the art is a catalyst able to absorb unburnedhydrocarbons. This catalyst has the property that the higher thesurrounding pressure, the greater the amount of absorption of unburnedhydrocarbons and that when the pressure of the surroundings becomeslower, the absorbed unburned hydrocarbons are released. Therefore, knownin the art is an internal combustion engine which utilizes this propertyto reduce the NO_(x) by the unburned hydrocarbons released from thecatalyst by arranging this catalyst in an engine exhaust passage,arranging an exhaust control valve inside the engine exhaust passagedownstream of the catalyst, injecting a small amount of auxiliary fuelduring an expansion stroke or exhaust stroke in addition to main fuelfor generating engine output at the time of engine low speed, low loadoperation when the amount of generation of NO_(x) is small, exhausting alarge amount of unburned hydrocarbons from the combustion chamber,closing the exhaust control valve to a relatively small opening degreeat this time so that the drop in engine output falls within theallowable range so as to raise the pressure inside the exhaust passageand cause a large amount of unburned hydrocarbons exhausted from thecombustion chamber to be absorbed in the catalyst, fully opening theexhaust control valve to cause the pressure in the exhaust passage tofall at the time of engine high speed or high load operation when theamount of generation of NO_(x) is large, and reducing the NO_(x) by theunburned hydrocarbons released from the catalyst at this time (seeJapanese Unexamined Patent Publication (Kokai) No. 10-238336).

Further, current diesel engines of course and also spark ignition typeinternal combustion engines have the major problem of how to reduce theamount of unburned hydrocarbons generated at the time of engine low loadoperation, in particular at the time of warmup operation of the engine.Therefore, the present inventors engaged in experiments and research tosolve this problem and as a result found that to greatly reduce theamount of unburned hydrocarbons exhausted into the atmosphere at thetime of warmup operation of an engine etc., it is necessary to reducethe amount of unburned hydrocarbons generated in the combustion chamberand simultaneously to increase the amount of reduction of unburnedhydrocarbons in the exhaust passage.

Specifically speaking, they learned that if auxiliary fuel isadditionally injected into the combustion chamber during the expansionstroke or exhaust stroke and that auxiliary fuel burned and if anexhaust control valve is provided in the engine exhaust passage aconsiderable distance away from the output of the engine exhaust portand the exhaust control valve is made to substantially completely close,the synergistic effect of the combustion of the auxiliary fuel and theexhaust throttling action due to the exhaust control valve causes theamount of generation of the unburned hydrocarbons in the combustionchamber to fall and the amount of unburned hydrocarbons in the exhaustpassage to rise and thereby enables the amount of unburned hydrocarbonsexhausted into the atmosphere to be greatly reduced.

Explaining this a bit more specifically, when auxiliary fuel isinjected, not only is the auxiliary fuel itself burned, but also theunburned hydrocarbons left over after burning the main fuel is burned inthe combustion chamber. Therefore, not only is the amount of unburnedhydrocarbons generated in the combustion chamber greatly reduced, butalso the unburned hydrocarbons remaining after burning the main fuel andthe auxiliary fuel are burned, so the temperature of the burned gasbecomes considerably high.

On the other hand, if the exhaust control valve is substantiallycompletely closed, the pressure in the exhaust passage from the exhaustport of the engine to the exhaust control valve, that is, the backpressure, becomes considerably high. A high back pressure means that thetemperature of the exhaust gas exhausted from the combustion chamberdoes not fall that much. Therefore, the temperature of the exhaust gasin the export port becomes considerably high. On the other hand, a highback pressure means that the flow rate of the exhaust gas exhausted inthe export port is slow. Therefore, the exhaust gas remains in a hightemperature state in the exhaust passage upstream of the exhaust controlvalve over a long period of time. During that time, the unburnedhydrocarbons contained in the exhaust. gas are oxidized and thereforethe amount of unburned hydrocarbons exhausted into the atmosphere isgreatly reduced.

In this case, if auxiliary fuel were not injected, the unburnedhydrocarbons left over after burning the main fuel remain as they are,so a large amount of unburned hydrocarbons is generated in thecombustion chamber. Further, if auxiliary fuel were not injected, thetemperature of the burned gas in the combustion chamber would not becomethat high, so even if substantially fully closing the exhaust controlvalve at this time, a sufficient action in purifying the unburnedhydrocarbons in the exhaust passage upstream of the exhaust controlvalve could not be expected. Therefore, at this time, a large amount ofunburned hydrocarbons would be exhausted into the atmosphere.

On the other hand, even if not throttling the exhaust by the exhaustcontrol valve, if injecting auxiliary fuel, the amount of unburnedhydrocarbons generated in the combustion chamber is reduced and thetemperature of the burned gas in the combustion chamber becomes high.When not throttling the exhaust by the exhaust control valve, however,the pressure of the exhaust gas immediately falls after the exhaust gasis exhausted from the combustion chamber and therefore the temperatureof the exhaust gas immediately falls. Therefore, in this case, almost noaction of oxidation of the unburned hydrocarbons in the exhaust passagecan be expected and therefore a large amount of unburned hydrocarbons isexhausted into the atmosphere at this time as well.

That is, to greatly reduce the amount of unburned hydrocarbons exhaustedinto the atmosphere, it becomes necessary to inject auxiliary fuel andsimultaneously substantially fully close the exhaust control valve.

In the diesel engine described in the above Japanese Unexamined PatentPublication (Kokai) No. 49-80414, no auxiliary fuel is injected and theamount of main fuel injected is greatly increased, so the temperature ofthe exhaust gas rises, but an extremely large amount of unburnedhydrocarbons is generated in the combustion chamber. If an extremelylarge amount of hydrocarbons is generated in the combustion chamber,even if there is an oxidation action of the unburned hydrocarbons in forexample the exhaust passage, only part of the unburned hydrocarbons willbe oxidized, so a large amount of unburned hydrocarbons will beexhausted into the atmosphere.

On the other hand, in the internal combustion engine described in theabove-mentioned Japanese Unexamined Patent Publication (Kokai) No.8-303290 or Japanese Unexamined Patent Publication (Kokai) No.10-212995, since there is no exhaust throttling action by the exhaustcontrol valve, almost no action in oxidizing the unburned hydrocarbonsin the exhaust passage can be expected. Therefore, even in this internalcombustion engine, a large amount of unburned hydrocarbons is exhaustedinto the atmosphere.

Further, in the internal combustion engine described in theabove-mentioned Japanese Unexamined Patent Publication (Kokai) No.10-238336, the exhaust control valve is closed to a relatively smallopening degree so that the drop in output of the engine falls within anallowable range and therefore in this internal combustion engine, theamount of main fuel injected is maintained at an identical amount ofinjection when the exhaust control valve is fully opened and when it isclosed. With an amount of closure of the exhaust control valve of anextent where the drop in engine output falls within an allowable range,however, the back pressure does not become that high.

Further, in this internal combustion engine, to generate the unburnedhydrocarbons to be absorbed in the catalyst, a small amount of auxiliaryfuel is injected into the expansion stroke or exhaust stroke. In thiscase, if the auxiliary fuel can be burned well, no unburned hydrocarbonsare generated any longer, so in this internal combustion engine, theinjection of the auxiliary fuel is controlled so that the auxiliary fuelis not burned well. Therefore, in this internal combustion engine, it isbelieved that a small amount of auxiliary fuel does not contribute thatmuch to the rise in temperature of the burned gas.

In this way, in this internal combustion engine, it is believed that alarge amount of unburned hydrocarbons is generated in the combustionchamber and further the back pressure does not become that high and thetemperature of the unburned gas does not rise that much, so the unburnedhydrocarbons are not oxidized that much even in the exhaust passage. Inthis internal combustion engine, the objective is to cause as large anamount of unburned hydrocarbons to be absorbed in the catalyst.Therefore, thinking in this way can be said to be rational.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust gaspurification device of an internal combustion engine able to ensurestable operation of the engine and greatly reduce the amount of unburnedhydrocarbons exhausted into the atmosphere.

According to the present invention, there is provided an exhaust gaspurification device of an internal combustion engine wherein an exhaustcontrol valve is arranged a predetermined distance away from an outletof an engine exhaust port inside an exhaust passage connected to theoutlet of the exhaust port; when it is judged that the amount ofunburned hydrocarbons exhausted into the atmosphere is to be reduced,the exhaust control valve is substantially fully closed and, in additionto burning the main fuel injected into the combustion chamber underexcess air to generate engine output, auxiliary fuel is additionallyinjected into the combustion chamber at a predetermined timing in theexpansion stroke or exhaust stroke where the auxiliary fuel can beburned so that the amount of unburned hydrocarbons produced in thecombustion chamber is reduced and the oxidizing reaction of hydrocarbonsin the exhaust port and the exhaust passage upstream of the exhaustcontrol valve is promoted; and when the exhaust control valve issubstantially fully closed, the amount of injection of main fuel isincreased compared with the case where the exhaust control valve isfully opened under the same engine operating conditions so as toapproach the torque generated by the engine when the exhaust controlvalve is fully opened under the same engine operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view of an internal combustion engine;

FIG. 2 is a side sectional view of a combustion chamber;

FIG. 3 is a view of an embodiment of an exhaust control valve;

FIG. 4 is a view of the amount of injection, injection timing, andair-fuel ratio;

FIG. 5 is a view of the injection timing;

FIG. 6 is a view of the concentration of unburned hydrocarbons;

FIG. 7 is a view of the amount of injection of main fuel;

FIG. 8 is a view of the relationship between the amount of injection ofmain fuel and the amount of injection of auxiliary fuel;

FIG. 9 is a view of the amount of injection of main fuel and the changein opening degree of the exhaust control valve;

FIG. 10 is a view of the amount of injection of main fuel and the changein opening degree of the exhaust control valve;

FIG. 11 is a flow chart of the operational control;

FIG. 12 is an overview of another embodiment of an internal combustionengine;

FIG. 13 is an overview of still another embodiment of an internalcombustion engine;

FIG. 14 is an overview of still another embodiment of an internalcombustion engine;

FIG. 15 is a flow chart of the operational control;

FIG. 16 is an overview of still another embodiment of an internalcombustion engine;

FIG. 17 is an overview of still another embodiment of an internalcombustion engine;

FIG. 18 is a flow chart of the operational control;

FIG. 19 is a flow chart of the operational control;

FIG. 20 is a flow chart of the operational control;

FIG. 21 is a view of the relation between the amount of depression ofthe accelerator pedal and the opening degree of the exhaust controlvalve;

FIG. 22 is a view of the amount of injection of the main fuel and theopening degree of the exhaust control valve;

FIG. 23 is an overview of still another embodiment of an internalcombustion engine;

FIG. 24 is a time chart of the change of the auxiliary fuel Qa;

FIG. 25 is a flow chart of the operational control;

FIG. 26 is a flow chart of execution of injection control of auxiliaryfuel;

FIG. 27 is a time chart of the change of the auxiliary fuel Qa;

FIG. 28 is a flow chart of execution of injection control of auxiliaryfuel;

FIG. 29 is a time chart of the change of the auxiliary fuel Qa;

FIG. 30 is a flow chart of execution of injection control of auxiliaryfuel;

FIG. 31 is an overview of still another embodiment of an internalcombustion engine;

FIG. 32 is an overview of still another embodiment of an internalcombustion engine;

FIG. 33 is an overview of still another embodiment of an internalcombustion engine;

FIG. 34 is a side sectional view of still another embodiment of aninternal combustion engine; and

FIG. 35 is a side sectional view of still another embodiment of aninternal combustion engine.

BEST MODE FOR WORKING THE INVENTION

FIG. 1 and FIG. 2 show the case of application of the present inventionto a stratified combustion type internal combustion engine. The presentinvention, however, can also be applied to a spark ignition typeinternal combustion engine wherein combustion is performed under auniform lean air-fuel ratio and a diesel engine where combustion isperformed under excess air.

Referring to FIG. 1, 1 indicates an engine body. The engine body 1 hasfour cylinders comprised of a no. 1 cylinder #1, no. 2 cylinder #2, no.3 cylinder #3, and no. 4 cylinder #4. FIG. 2 is a side sectional view ofeach of the cylinders #1, #2, #3, and #4. Referring to FIG. 2, 2 is acylinder block, 3 a cylinder head, 4 a piston, 5 a combustion chamber, 6a fuel injector arranged at the edge of the inner wall of the cylinderhead 3, 7 a spark plug arranged at the center of the inner wall of thecylinder head 3, 8 an intake valve, 9 an intake port, 10 an exhaustvalve, and 11 an exhaust port.

Referring to FIG. 1 and FIG. 2, the intake port 9 is connected to asurge tank 13 through a corresponding intake tube 12, while the surgetank 13 is connected to an air cleaner 16 through an intake duct 14 andair flow meter 15. Inside the intake duct 14 is arranged a throttlevalve 18 driven by a step motor 17. On the other hand, in the embodimentshown in FIG. 1, the firing order is made 1-3-4-2. As shown in FIG. 1,the exhaust ports 11 of the cylinders #1 and #4 of every other positionin the firing order are connected to a common first exhaust manifold 19,while the exhaust ports 11 of the remaining cylinders #2 and #3 of everyother position in the firing order are connected to a common secondexhaust manifold 20. These first exhaust manifold 19 and second exhaustmanifold 20 are connected to a common exhaust pipe 21. The exhaust pipe21 is connected to a separate exhaust pipe 22. Inside the exhaust pipe22 is arranged an exhaust control valve 24 driven by an actuator 23comprised of a vacuum operated diaphragm device or electric motor.

As shown in FIG. 1, the exhaust pipe 21 and surge tank 13 are connectedto each other through an exhaust gas recirculation (hereinafter referredto as “EGR”) passage 25. Inside the EGR passage 25 is arranged anelectrically controlled EGR control valve 26. The fuel injector 6 isconnected to a common fuel reservoir, that is, a so-called common rail27. The fuel inside the fuel tank 28 is supplied into the common rail 27through an electrically controlled variable discharge fuel pump 29. Thefuel supplied in the common rail 27 is supplied to each fuel injector 6.The common rail 27 has a fuel pressure sensor 30 attached to it fordetecting the fuel pressure in the common rail 27. The discharge of thefuel pump 29 is controlled based on the output signal of the fuelpressure sensor 30 so that the fuel pressure in the common rail 27becomes a target fuel pressure.

An electronic control unit 40 is comprised of a digital computerprovided with a ROM (read only memory) 42, RAM (random access memory)43, CPU (microprocessor) 44, input port 45, and output port 46 connectedto each other through a bidirectional bus 41. The air flow meter 15generates an output voltage proportional to the amount of intake air.Its output voltage is input to the input port 45 through thecorresponding AD converter 47. The engine body 1 has a water temperaturesensor 31 attached to it for detecting the engine coolant watertemperature. The output signal of the water temperature sensor 31 isinput to the input port 45 through a corresponding AD converter 47.Further, the input port 45 receives as input the output signal of thefuel pressure sensor 30 through the corresponding AD converter 47.

Further, an accelerator pedal 50 has connected to it a load sensor 51generating an output voltage proportional to the amount of depression Lof the accelerator pedal 50. The output voltage of the load sensor 51 isinput to the input port 45 through the corresponding AD converter 47.Further, the input port 45 has connected to it a crank angle sensor 52generating an output pulse each time a crankshaft rotates by for example30 degrees. On the other hand, the output port 46 is connected throughcorresponding drive circuits 48 to the fuel injectors 6, the spark plugs7, the step motor 17 for driving the throttle valve, the actuator 23 forcontrolling the exhaust control valve, the EGR control valve 26, and thefuel pump 29.

FIG. 4 shows the amounts of fuel injection Q1, Q2, and Q (=Q₁+Q₂), theinjection start timings θS1 and θS2, injection end timings θE1 and θE2,and mean air-fuel ratio A/F in the combustion chamber 5. Note that inFIG. 4, the abscissa L shows the amount of depression of the acceleratorpedal 50, that is, the required load.

As will be understood from FIG. 4, when the required load L is lowerthan L₁, the fuel injection Q2 is performed between θS2 and θE2 at theend of the compression stroke. At this time, the mean air-fuel ratio A/Fbecomes considerably lean. When the required load L is between L₁ andL₂, the first fuel injection Q1 is performed between θS1 and θE1 of thestart of the suction stroke, then the second fuel injection Q2 isperformed between θS2 and θE2 of the end of the compression stroke. Atthis time as well, the air-fuel ratio A/F becomes lean. When therequired load is larger than L₂, the fuel injection Q1 is performedbetween θS1 and θE1 at the start of the suction stroke. At this time,when the required load L is in the low region, the mean air-fuel ratioA/F is made lean, when the required load becomes high, the mean air-fuelratio A/F is made the stoichiometric air-fuel ratio, while when therequired load L becomes further higher, the mean air-fuel ratio A/F ismade rich. Note that the operating region where the fuel injection Q2 isperformed only at the end of the compression stroke, the operatingregion where the fuel injections Q1 and Q2 are performed twice, and theoperating region where the fuel injection Q1 is performed only at thestart of the suction stroke are not determined by. just the requiredload L and are in actuality determined by the required load L and theengine speed.

FIG. 2 shows the case where the fuel injection Q2 is performed only whenthe required load L is smaller than L₁ (FIG. 4), that is, at the end ofthe compression stroke. As shown in FIG. 2, a cavity 4 a is formed inthe top surface of the piston 4. When the required load L is lower thanL₁, fuel is injected from the fuel injector 6 toward the bottom wall ofthe cavity 4 a. This fuel is guided by the peripheral wall of the cavity4 a and heads toward the spark plug 7. Due to this, an air-fuel mixtureG is formed around the spark plug 7. Next, this air-fuel mixture G ismade to ignite by the spark plug 7.

On the other hand, when the required load is between L₁ and L₂ asexplained above, the fuel injection is performed divided into two. Inthis case, a lean air-fuel mixture is formed in the combustion chamber 5by the first fuel injection Q1 performed at the start of the suctionstroke. Next, an air-fuel mixture of an optimal concentration is formedaround the spark plug 7 by the second fuel injection Q2 performed at theend of the compression stroke. This air-fuel mixture is ignited by thespark plug 7. Due to the ignition flame, the lean air-fuel mixture isburned.

On the other hand, when the required load L is larger than L₂, as shownin FIG. 4, a uniform air-fuel mixture of a lean or stoichiometricair-fuel ratio or rich air-fuel ratio is formed inside the combustionchamber 5. This uniform air-fuel mixture is burned by the spark plug 7.

Next, a general explanation will first be given of the method ofreducing the unburned hydrocarbons according to the present inventionwhile referring to FIG. 5. Note that in FIG. 5, the abscissa shows thecrank angle, while BTDC and ATDC show before top dead center and aftertop dead center.

FIG. 5(A) shows the fuel injection timing when there is no particularneed to reduce the unburned hydrocarbons by the method according to thepresent invention and the required load L is smaller than L₁. As shownin FIG. 5(A), at this time, only the main fuel Qm is injected at the endof the compression stroke. At this time, the exhaust control valve 24 isheld in the fully opened state.

As opposed to this, when it is necessary to reduce the unburnedhydrocarbons by the method according to the present invention, theexhaust control valve 24 is substantially fully closed. Further, asshown in FIG. 5(B), auxiliary fuel Qa is additionally injected duringthe expansion stroke, in the example shown in FIG. 5(B), near 60° aftercompression top dead center (ATDC), in addition to the injection of themain fuel Qm for generating the engine output. Note that in this case,the main fuel Qm is burned under excess air so that sufficient oxygenremains in the combustion chamber 5 for completely burning the auxiliaryfuel Qa after burning the main fuel Qm. Further, FIG. 5(A) and FIG. 5(B)show the fuel injection timing when the engine load and engine speed arethe same. Therefore, when the engine load and engine speed are the same,the amount of injection of the main fuel Qm in the case shown in FIG.5(B) is increased compared with the amount of injection of main fuel Qmin the case shown in FIG. 5(A).

FIG. 6 shows an example of the concentration (ppm) of unburnedhydrocarbons in the exhaust gas at different positions of the engineexhaust passage. In the example shown in FIG. 6, the black triangleshows the concentration (ppm) of the unburned hydrocarbons in theexhaust gas at the exhaust port 11 outlet in the case of injecting themain fuel Qm at the end of the compression stroke as shown in FIG. 5(A)in the state where the exhaust control valve 24 is fully closed. In thiscase, the concentration of the unburned hydrocarbons in the exhaust gasat the exhaust port 11 outlet becomes an extremely high value of atleast 6000 ppm.

On the other hand, in the example shown in FIG. 6, the black dots andthe solid line show the concentration (ppm) of the unburned hydrocarbonsin the exhaust gas when substantially fully closing the exhaust controlvalve 24 and injecting main fuel Qm and auxiliary fuel Qa as shown inFIG. 5(B). In this case, the concentration of the unburned hydrocarbonsin the exhaust gas at the exhaust port 11 outlet becomes not more than2000 ppm. Near the exhaust control valve 24, the concentration of theunburned hydrocarbons in the exhaust gas falls to about 150 ppm.Therefore, in this case, it is learned that the amount of the unburnedhydrocarbons exhausted into the atmosphere is greatly reduced.

The reason why the unburned hydrocarbons are reduced in the exhaustpassage upstream of the exhaust control valve 24 in this way is that theoxidation reaction of the unburned hydrocarbons is promoted. As shown bythe black triangle of FIG. 6, when the amount of the unburnedhydrocarbons at the exhaust port 11 outlet is large, that is, when theamount of generation of unburned. hydrocarbons in the combustion chamber5 is large, even if the oxidation reaction of the unburned hydrocarbonsin the exhaust passage is promoted, the amount of the unburnedhydrocarbons exhausted into the atmosphere does not fall that much. Thatis, the amount of unburned hydrocarbons exhausted into the atmospherecan be greatly reduced by the promotion of the oxidation reaction of theunburned hydrocarbons in the exhaust passage when the concentration ofunburned hydrocarbons at the exhaust port 11 outlet is low, that is, theamount of generation of unburned hydrocarbons in the combustion chamber5 is small, as shown by the black dots of FIG. 6.

To reduce the amount of unburned hydrocarbons exhausted into theatmosphere in this way, it is necessary to simultaneously satisfy thetwo requirements of reducing the amount of generation of unburnedhydrocarbons in the combustion chamber 5 and promoting the oxidationreaction of the unburned hydrocarbons in the exhaust passage. Therefore,first, an explanation will be given of the second requirement, that is,the promotion of the oxidation reaction of the unburned hydrocarbons inthe exhaust passage.

According to the present invention, the exhaust control valve 24 issubstantially fully closed when the amount of unburned hydrocarbonsexhausted into the atmosphere should be reduced. When the exhaustcontrol valve 24 is substantially fully closed in this way, the pressureinside the exhaust port 11, inside the exhaust manifolds 19 and 20,inside the exhaust pipe 21, and inside the exhaust pipe 22 upstream ofthe exhaust control valve 24, that is, the back pressure, becomesconsiderably high. The fact that the back pressure becomes high meansthat the pressure of the exhaust gas will not fall that much when theexhaust gas is exhausted into the exhaust port 11 from the combustionchamber 5 and therefore the temperature of the exhaust gas exhaustedfrom the combustion chamber 5 will also not fall that much. Therefore,the temperature of the exhaust gas exhausted into the exhaust port 11 ismaintained at a considerably high temperature. On the other hand, thefact that the back pressure is high means that the density of theexhaust gas is high. The fact that the density of the exhaust gas ishigh means that the flow rate of the exhaust gas in the exhaust passagefrom the exhaust port 11 to the exhaust control valve 24 is slow.Therefore, the exhaust gas exhausted into the exhaust port 11 remains inthe exhaust passage upstream of the exhaust control valve 24 under ahigh temperature over a long time period.

When the exhaust gas is made to remain in the exhaust passage upstreamof the exhaust control valve 24 under a high temperature for a long timeperiod, the oxidation reaction of the unburned hydrocarbons is promotedduring that time. In this case, according to experiments by the presentinventors, it was found that to promote the oxidation reaction of theunburned hydrocarbons in the exhaust passage, it is necessary to makethe temperature of the exhaust gas at the exhaust port 11 outlet atleast about 750° C., preferably at least 800° C.

Further, the longer the time where the high temperature exhaust gasremains in the exhaust passage upstream of the exhaust control valve 24,the greater the amount of reduction of unburned hydrocarbons. The timewhere it remains there becomes longer the further the position of theexhaust control valve 24 from the exhaust port 11 outlet. Therefore, theexhaust control valve 24 has to be arranged away from the exhaust port11 outlet by a distance necessary for sufficiently reducing the unburnedhydrocarbons. If arranging the exhaust control valve 24 away from theexhaust port 11 outlet by a distance necessary for sufficiently reducingthe unburned hydrocarbons, the concentration of unburned hydrocarbons isgreatly reduced as shown by the solid line in FIG. 6. Note thataccording to experiments by the present inventors, it was found that tosufficiently reduce the unburned hydrocarbons, it is preferable to makethe distance from the exhaust port 11 outlet to the exhaust controlvalve 24 at least 30 cm.

To promote the oxidation reaction of the unburned hydrocarbons in theexhaust passage as explained above, however, it is necessary to make thetemperature of the exhaust gas at the exhaust port 11 outlet at leastabout 750° C., preferably at least 800° C. Further, to reduce the amountof unburned hydrocarbons exhausted into the atmosphere, it is necessaryto satisfy the first requirement explained above. That is, it isnecessary to reduce the amount of generation of unburned hydrocarbons inthe combustion chamber 5. Therefore, in the present invention, inaddition to the main fuel Qm for generating the engine output, theauxiliary fuel Qa is additionally injected after the injection of themain fuel Qm and the auxiliary fuel Qa burned in the combustion chamber5.

That is, if the auxiliary fuel Qa is burned in the combustion chamber 5,the large amount of unburned hydrocarbons remaining after burning themain fuel Qm is burned at the time of burning the auxiliary fuel Qa.Further, this auxiliary fuel Qa is injected into the high temperaturegas, so the auxiliary fuel Qa is burned well. Therefore, the unburnedhydrocarbons remaining after burning the auxiliary fuel Qa is no longergenerated that much. Therefore, the amount of the unburned hydrocarbonsfinally generated in the combustion chamber becomes considerably small.

Further, if the auxiliary fuel Qa is burned in the combustion chamber 5,in addition to the heat due to the combustion of the main fuel Qm itselfand the auxiliary fuel Qa itself, the heat of combustion of the unburnedhydrocarbons remaining after burning the main fuel Qm is additionallygenerated, so the temperature of the burned gas in the combustionchamber 5 becomes considerably high. By additionally injecting auxiliaryfuel Qa and burning the auxiliary fuel Qa in addition to the main fuelQm, it is possible to reduce the amount of the unburned hydrocarbonsgenerated in the combustion chamber 5 and make the temperature of theexhaust gas in the exhaust port 11 outlet at least 750° C., preferablyat least 800° C.

In this way, in the present invention, it is necessary to burn theauxiliary fuel Qa in the combustion chamber 5. Therefore, it isnecessary that sufficient oxygen remain in the combustion chamber 5 atthe time of combustion of the auxiliary fuel Qa. Further, it isnecessary to inject auxiliary fuel Qa at the timing when the injectedauxiliary fuel Qa would be burned well in the combustion chamber 5.

Therefore, in the present invention, as explained above, the main fuelQm is burned under excess air so that sufficient oxygen can remain inthe combustion chamber 5 at the time of combustion of the auxiliary fuelQa. At this time, the auxiliary fuel Qa is also burned under excess air.In this case, it was found that the mean air-fuel ratio in thecombustion chamber 5 at the time of combustion of the main fuel Qm ispreferably at least about 30, while the mean air-fuel ratio in thecombustion chamber 5 at the time of combustion of the auxiliary fuel Qais preferably at least about 15.5.

Further, in a stratified combustion type internal combustion engineshown in FIG. 2, the injection timing when the auxiliary fuel Qainjected would burn well in the combustion chamber 5 is an expansionstroke from about 50° to about 90° after compression top dead center(ATDC) shown by the arrow mark Z in FIG. 5. Therefore, in a stratifiedcombustion type internal combustion engine shown in FIG. 2, theauxiliary fuel Qa is injected in the expansion stroke from about 50° toabout 90° after compression top dead center (ATDC). Note that theauxiliary fuel Qa injected in the expansion stroke of about 50° to about90° after compression top dead center (ATDC) does not contribute thatmuch to the generation of the engine output.

According to experiments of the present inventors, however, in astratified combustion type internal combustion engine shown in FIG. 2,when auxiliary fuel Qa is injected from 60° to 70° after compression topdead center (ATDC), the amount of the unburned hydrocarbons exhaustedinto the atmosphere becomes the smallest. Therefore, in this embodimentaccording to the present invention, as shown in FIG. 5(B), the injectiontiming of the auxiliary fuel Qa is made near about 60° after compressiontop dead center (ATDC).

The optimal injection timing of the auxiliary fuel Qa differs dependingon the type of the engine. For example, in a diesel engine, the optimalinjection timing for the auxiliary fuel Qa is during the expansionstroke or during the exhaust stroke. Therefore, in the presentinvention, the injection of the auxiliary fuel Qa is carried out in theexpansion stroke or the exhaust stroke.

On the other hand, the temperature of the burned gas in the combustionchamber 5 is influenced by both of the heat of combustion of the mainfuel Qm and the heat of combustion of the auxiliary fuel Qa. That is,the temperature of the burned gas in the combustion chamber 5 becomeshigher the greater the amount of injection of the main fuel Qm andbecomes higher the greater the amount of injection of the auxiliary fuelQa. Further, the temperature of the burned gas in the combustion chamber5 is influenced by the back pressure. That is, the higher the backpressure, the harder it is for the burned gas to flow out from theinside of the combustion chamber 5, so the greater the amount of burnedgas remaining in the combustion chamber 5. Therefore, if the exhaustcontrol valve 24 is substantially fully closed, the temperature of theburned gas in the combustion chamber 5 is raised.

If the exhaust control valve 25 is substantially fully closed, however,and therefore the back pressure becomes higher, even if auxiliary fuelQa were additionally injected, the torque generated by the engine wouldfall from the optimum required generated torque. Therefore, in thepresent invention, when the exhaust control valve 24 is substantiallyfully closed such as shown in FIG. 5(B), the amount of injection of themain fuel Qm is increased compared with the case where the exhaustcontrol valve 24 is fully opened under the same engine operating stateso as to approach the required generated torque of the engine when theexhaust control valve 24 is fully opened under the same engine operatingstate as shown in FIG. 5(B). Note that in this embodiment of the presentinvention, when the exhaust control valve 24 is substantially fullyclosed, the main fuel Qm is increased so that the torque generated bythe engine at that time matches the required generated torque of theengine when the exhaust control valve 24 is fully opened under the sameengine operating state.

FIG. 7 shows the change in the main fuel. Qm required for obtaining therequired generated torque of the engine with respect to the requiredload L. Note that in FIG. 7, the solid line shows the case where theexhaust control valve 24 is substantially fully closed, while the brokenline shows the case where the exhaust control valve 24 is fully opened.

On the other hand, FIG. 8 shows the relationship of the main fuel Qm andauxiliary fuel Qa required for making the temperature of the exhaust gasat the exhaust port 11 outlet about 750° C. to about 800° C. whensubstantially fully closing the exhaust control valve 24. As explainedabove, if increasing the main fuel Qm, the temperature of the burned gasin the combustion chamber 5 becomes higher, while if the auxiliary fuelQa is increased, the temperature of the burned gas in the combustionchamber 5 becomes higher. Therefore, the relationship between the mainfuel Qm and auxiliary fuel Qa required for making the temperature of theexhaust gas at the exhaust port 11 outlet from about 750° C. to about800° C. becomes one as shown in FIG. 8, where if increasing the mainfuel Qm, the auxiliary fuel Qa is decreased, while if decreasing themain fuel Qm, the auxiliary fuel Qa is increased.

If increasing the main fuel Qm and auxiliary fuel Qa by the same amount,however, the amount of rise of temperature inside the combustion chamber5 becomes far greater in the case of increasing the auxiliary fuel Qathan the case of increasing the main fuel Qm. Therefore, seen from theviewpoint of the reduction of the amount of fuel consumption, it can besaid to be preferable to raise the temperature of the burned gas in thecombustion chamber 5 by increasing the auxiliary fuel Qa.

Therefore, in this embodiment of the present invention, whensubstantially closing the exhaust control valve 24, the main fuel Qm isincreased by exactly the amount required for raising the torquegenerated by the engine to the required generated torque so as to raisethe temperature of the burned gas in the combustion chamber 5 due to theheat of combustion of mainly the auxiliary fuel Qa.

If substantially closing the exhaust control valve 24 and injecting theamount of auxiliary fuel Qa required for making the exhaust gas in theexhaust port 11 outlet at least about 750° C., preferably at least about800° C., the concentration of the unburned hydrocarbons can be greatlyreduced in the exhaust passage from the exhaust port 11 to the exhaustcontrol valve 24. At this time, to reduce the concentration of theunburned hydrocarbons down to about 150 ppm as shown in FIG. 6 in theexhaust passage from the exhaust port 11 to the exhaust control valve24, it is necessary to make the pressure in the exhaust passage upstreamof the exhaust control valve 24 from about 60 KPa to 80 KPa by gaugepressure. The rate of closure of the sectional area of the exhaustpassage by the exhaust control valve 24 at this time is about 95percent.

Therefore, in the embodiment shown in FIG. 1, when greatly reducing theamount of exhaust of unburned gas into the atmosphere, the exhaustcontrol valve 24 is substantially fully closed so that the rate ofclosure of the sectional area of the exhaust passage by the exhaustcontrol valve 24 becomes about 95 percent. Note that in this case, asshown in FIG. 3, it is possible to make a through hole 24 a in the valveelement of the exhaust control valve 24 and completely close the exhaustcontrol valve 24.

On the other hand, when it is sufficient to reduce the unburnedhydrocarbons from 600 ppm to about 800ppm in the exhaust passage fromthe export port 11 to the exhaust control valve 24, it is sufficient tomake the pressure of the exhaust passage upstream of the exhaust controlvalve 24 about 30 KPa by gauge pressure. The rate of closure of thesectional area of the exhaust passage by the exhaust control valve 24 atthis time becomes about 90 percent.

A large amount of unburned hydrocarbons is generated at the internalcombustion engine when the temperature of the combustion chamber 5 islow. The times when the temperature in the combustion chamber 5 is loware the time of engine startup and warmup operation and the time ofengine low load. Therefore, at the time of engine startup and warmupoperation and the time of engine low load, a large amount of unburnedhydrocarbons is generated. When the temperature in the combustionchamber 5 is low in this way, even if arranging a catalyst having anoxidation function in the exhaust passage, it is difficult to oxidizethe large amount of unburned hydrocarbons generated by a catalyst exceptwhen the catalyst becomes over an activation temperature.

Therefore, in this embodiment of the present invention, at the time ofengine startup and warmup operation and the time of engine low load, theexhaust control valve 24 is substantially fully closed, the main fuel Qmis increased, the auxiliary fuel Qa is additionally injected, andtherefore the amount of unburned hydrocarbons exhausted into theatmosphere is greatly reduced.

FIG. 9 shows an example of the change of the main fuel Qm at the time ofengine startup and warmup operation and the change in the opening degreeof the exhaust control valve 24. Note that in FIG. 9, the solid line Xshows the optimum amount of injection of the main fuel Qm whensubstantially fully closing the exhaust control valve 24, while thebroken line Y shows the optimum amount of injection of main fuel Qm whenfully opening the exhaust control valve 24. As will be understood fromFIG. 9, when the engine is started, the exhaust control valve 24 isswitched from the fully opened state to the substantially fully closedstate, the amount of injection X of the main fuel Qm is increased fromthe optimum amount of injection Y of the main fuel Qm when the exhaustcontrol valve 24 is fully opened under the same engine operatingconditions, and the auxiliary fuel Qa is additionally injected.

FIG. 10 shows an example of the change of the main fuel Qm at the timeof engine low load and the change in the opening degree of the exhaustcontrol valve 24. Note that in FIG. 10, the solid line X shows theoptimum amount of injection of the main fuel Qm when substantially fullyclosing the exhaust control valve 24, while the broken line Y shows theoptimum amount of injection of main fuel Qm when fully opening theexhaust control valve 24. As will be understood from FIG. 10, at thetime of engine low load, the exhaust control valve 24 is substantiallyfully closed, the amount of injection X of the main fuel Qm is increasedfrom the optimum amount of injection Y of the main fuel Qm when theexhaust control valve 24 is fully opened under the same engine operatingconditions, and the auxiliary fuel Qa is additionally injected.

FIG. 11 shows a routine of the operational control.

Referring to FIG. 11, first, at step 100, it is determined if the engineis starting up and in warmup operation. When the engine is not startingup and in warmup operation, the routine jumps to step 102, where it isdetermined if the engine is operating under low load. When the engine isnot operating under low load, the routine proceeds to step 103, wherethe exhaust control valve 24 is fully opened, then the routine proceedsto step 104, where the injection of the main fuel Qm is controlled. Atthis time, the auxiliary fuel Qa is not injected.

On the other hand, when it is determined at step 100 that the engine isstarting up and in warmup operation, the routine proceeds to step 101,where it is determined if a predetermined set time has elapsed fromengine startup. When a set time has not elapsed, the routine proceeds tostep 105. On the other hand, when the set time has elapsed, the routineproceeds to step 102. When it is determined at step 102 that the engineis operating. under low load, the routine proceeds to step 105. At step105, the exhaust control valve 24 is substantially fully closed, then atstep 106, the injection of the main fuel Qm is controlled. That is, ifthe engine is starting up and in warmup operation, the amount ofinjection of the main fuel Qm is made the X shown in FIG. 9. If theengine is operating under low load, the amount of injection of the mainfuel Qm is made the X shown in FIG. 10. Next, at step 107, the injectionof the auxiliary fuel Qa is controlled.

FIG. 12 shows the case of use of a vacuum operated type actuator as theactuator 23. Note that in the example shown in FIG. 12, as the vacuumoperated type actuator, use is made of a vacuum operated diaphragmdevice comprised of a diaphragm 60 connected to the exhaust controlvalve 24, a diaphragm vacuum chamber 61, and a diaphragm pressingcompression spring 62. Further, the vacuum tank 63 is on the one handconnected to the inside of the surge tank 13 through a check valve 64enabling flow only toward the surge tank 13 and on the other handconnected to the diaphragm vacuum chamber 61 through a changeover valve65 able to communicate with the atmosphere.

If the level of vacuum in the surge tank 13 becomes larger than thelevel of vacuum in the vacuum tank 63, the check valve 64 opens andtherefore the inside of the vacuum tank 63 is maintained at the maximumlevel of vacuum generated in the surge tank 13. When the diaphragmvacuum chamber 61 is opened to the atmosphere by the switching action ofthe changeover valve 65, the exhaust control valve 24 is fully opened.When the diaphragm vacuum chamber 61 is connected to the inside of thevacuum tank 63 due to the switching action of the changeover valve 65,the exhaust control valve 24 is substantially fully closed.

When the engine is stopped, the exhaust control valve 24 is held in thefully opened state so as not to be fixed in the closed state. Next, atthe time of engine startup, the exhaust control valve 24 is switchedfrom the fully opened state to the substantially fully closed state. Inthe example shown in FIG. 12, even at the time when the engine isstopped, vacuum is accumulated in the vacuum tank 63. Therefore, at thetime of engine startup, by connecting the diaphragm vacuum chamber 61 tothe vacuum tank 63, the exhaust control valve 24 can be reliablyswitched from the fully opened state to the substantially fully closedstate.

FIG. 13 shows another embodiment. In this embodiment, a catalyst 70 isarranged in the exhaust pipe 22 upstream of the exhaust control valve24. When a catalyst 70 is arranged in the exhaust pipe 22 upstream ofthe exhaust control valve 24 in this way, if auxiliary fuel Qa isadditionally injected and the exhaust control valve 24 is substantiallyfully closed, the catalyst 70 is strongly heated by the high temperatureexhaust gas. Therefore, at the time of engine startup and warmupoperation, the catalyst 70 can be activated early.

As the catalyst 70 arranged in the exhaust pipe 22, it is possible touse an oxidation catalyst, three-way catalyst, NO_(x) absorbent, orhydrocarbon absorbinq catalyst. The NO_(x) absorbent has the function ofabsorbing the NO_(x) when the mean air-fuel ratio in the combustionchamber 5 is lean and releasing the NO_(x) when the mean air-fuel ratioin the combustion chamber 5 becomes rich.

The NO_(x) absorbent is for example comprised of alumina as a carrierand carries on the carrier for example at least one of an alkali metalsuch as potassium K, sodium Na, lithium Li, and cesium Cs, an alkaliearth such as barium Ba and calcium Ca, and a rare earth such aslanthanum La and yttrium Y and a precious metal such as platinum Pt.

On the other hand, the hydrocarbon absorbing catalyst is for examplecomprised of a porous carrier such as zeolite, alumina Al₂O₃, silicaalumina SiO₂·Al₂O₃, activated carbon, and titania TiO₂ on which iscarried a precious metal such as platinum Pt, palladium Pd, rhodium Rh,and iridium Ir or a transition metal such as copper Cu, iron Fe, cobaltCo, and nickel Ni.

In such a hydrocarbon absorbing catalyst, the unburned hydrocarbons inthe exhaust gas are physically absorbed in the catalyst. The amount ofabsorption of the unburned hydrocarbons increases the lower thetemperature of the catalyst and increases the higher the pressure of theexhaust gas flowing through the catalyst. Therefore, in the embodimentshown in FIG. 13, when the temperature of the catalyst 70 is low and theback pressure is increased due to the exhaust throttling action of theexhaust control valve 24, that is, at the time of engine startup andwarmup operation and at the time of engine low load operation, theunburned hydrocarbons contained in the exhaust gas are absorbed in thehydrocarbon absorbing catalyst. Therefore, it is possible to furtherreduce the amount of unburned hydrocarbons exhausted into theatmosphere. Note that the unburned hydrocarbons absorbed in thehydrocarbon absorbing catalyst are released from the hydrocarbonabsorbing catalyst when the back pressure becomes low or when thetemperature of the hydrocarbon absorbing catalyst becomes higher.

FIG. 14 shows still another embodiment. In this embodiment, the catalyst70 comprised of an NO_(x) absorbent or hydrocarbon absorbing catalyst isarranged in the exhaust pipe 22 upstream of the exhaust control valve24, while a catalyst 71 or 72 having an oxidation function such as anoxidation catalyst or three-way catalyst is arranged between the firstexhaust manifold 19 and exhaust pipe 21 and the second exhaust manifold20 and exhaust pipe 21. When the exhaust control valve 24 issubstantially fully closed and the auxiliary fuel Qa is injected, thetemperature of the exhaust gas at the outlets of the exhaust manifolds19 and 20 is considerably high. Therefore, if catalysts 71 and 72 arearranged at the outlets of the exhaust manifolds 19 and 20, thesecatalysts 71 and 72 are activated an early time after engine startup. Asa result, the amount of unburned hydrocarbons exhausted into theatmosphere is further reduced due to the action of the catalysts 71 and72 on promoting the oxidation reaction.

As shown in FIG. 14, however, when arranging catalysts 71 and 72 havingoxidation functions in the engine exhaust passage, even at the time ofengine low load operation, the catalysts 71 and 72 are held at over theactivation temperature so long as the engine low load operation does notcontinue for a long time. Further, when the engine is restarted in ashort time after the engine stops, sometimes the catalysts 71 and 72 areheld at above the activation temperature even though the engine is inwarmup operation. If the catalysts 71 and 72 are activated, the unburnedhydrocarbons in the exhaust gas are purified by the catalysts 71 and 72.Therefore, it is no longer necessary to inject auxiliary fuel Qainviting an increase in the amount of fuel consumption.

Therefore, in a further embodiment, as shown in FIG. 14, catalysts 71and 72 are attached to the temperature sensors 73 and 74 for detectingthe temperatures of the catalysts 71 and 72 as shown in FIG. 14. Wheneither of the catalysts 71 and 72 reaches more than the activationtemperature based on the output signals of the temperature sensors 73and 74, even at the time of warmup operation or engine low loadoperation, the exhaust control valve 24 is fully opened and theinjection of the auxiliary fuel Qa is stopped.

FIG. 15 shows a routine for operational control in such a case.

Referring to FIG. 15, first, at step 200, it is determined if the engineis starting up and in warmup operation. When the engine is not startingup and in warmup operation, the routine proceeds to step 201, where itis determined if the engine is operating under low load. When the engineis not operating under low load, the routine proceeds to step 202, wherethe exhaust control valve 24 is fully opened, then the routine proceedsto step 203, where the injection of the main fuel Qm is controlled. Atthis time, the auxiliary fuel Qa is not injected.

On the other hand, if it is judged at step 200 that the engine isstarting up and in warmup operation or if it is determined at step 201that the engine is operating under low load, the routine proceeds tostep 204, where it is determined if the temperature T1 of the catalyst71 detected by the temperature sensor 73 and the temperature T2 of thecatalyst 72 detected by the temperature sensor 74 are higher than theactivation temperature T₀. When T1≦T₀ or T2≦T₀, the routine proceeds tostep 205, where the exhaust control valve 24 is substantially fullyclosed, then at step 206, the injection of the main fuel Qm iscontrolled. That is, at the time of engine startup and warmup operation,the amount of injection of the main fuel Qm is made the X shown in FIG.9, while at the time of engine low load operation, the amount ofinjection of the main fuel Qm is made the X shown in FIG. 10. Next, atstep 207, the injection of the auxiliary fuel Qa is controlled.

As opposed to this, when it is judged at step 204 that T1>T₀ and T2>T₀,that is, when both of the catalysts 71 and 72 are activated, the routineproceeds to step 202, where the exhaust control valve 24 is fully openedand then the routine proceeds to step 203, where the injection of themain fuel Qm is controlled.

On the other hand, as explained above, to greatly reduce the amount ofunburned hydrocarbons exhausted into the atmosphere, it is necessary tomake the temperature of the exhaust gas at the exhaust port 11 outlet atleast about 750° C. Therefore, it is necessary to maintain the backpressure at about 60 KPa to 80 KPa. However, there is a danger thatdeposits in the exhaust pipe 22 will prevent the exhaust control valve24 from closing up to the target opening degree and as a result the backpressure from becoming sufficiently high. Further, even if the exhaustcontrol valve 24 closes up to the target opening degree, the area of theflow channel of the exhaust gas becomes smaller due to the deposits andas a result there is the danger that the back pressure will become toohigh.

Therefore, in the embodiment explained below, when the amount of exhaustof the unburned hydrocarbons into the atmosphere should be reduced, thecombustion in the combustion chamber 5 is controlled so that thepressure or temperature of the exhaust gas in the exhaust passageupstream of the exhaust control valve 24 becomes the target value.Specifically speaking, if the amount of injection of at least one of themain fuel Qm and auxiliary fuel Qa is increased, the combustion pressureand combustion temperature in the combustion chamber 5 becomes higherand therefore the back pressure and temperature of the exhaust gasrises. Further, if the amount of intake air increases, the amount ofexhaust gas increases, so the back pressure and temperature of theexhaust gas rise.

Therefore, in the embodiment shown in FIG. 16, a pressure sensor 80 fordetecting the back pressure is mounted in the exhaust pipe 22. When theback pressure is lower than a target value, the amount of injection ofthe main fuel Qm, the amount of injection of the auxiliary fuel Qa, orthe amount of intake air is increased, while when the back pressure ishigher than the target value, the amount of injection of the main fuelQm, the amount of injection of the auxiliary fuel Qa, or the amount ofintake air is decreased.

Further, in the embodiment shown in FIG. 17, a temperature sensor 81 fordetecting the temperature of the exhaust gas at the exhaust port 11outlet is attached in the tube of the first exhaust manifold 19. Whenthe temperature of the exhaust gas detected by the temperature sensor 81is lower than a target value, the amount of injection of the main fuelQm, the amount of injection of the auxiliary fuel Qa, or the amount ofintake air is increased, while when the temperature of the exhaust gasdetected by the temperature sensor 81 is higher than the target value,the amount of injection of the main fuel Qm, the amount of injection ofthe auxiliary fuel Qa, or the amount of intake air is decreased.

Note that the exhaust control valve 24 can be arranged in the inlet ofthe exhaust pipe 22 as shown in FIG. 17 and can be arranged in theoutlet of the exhaust pipe 21.

FIG. 18 shows the routine for operational control of the case whencontrolling the back pressure by controlling the main fuel Qm.

Referring to FIG. 18, first, at step 300, it is determined if the engineis starting up and in warmup operation. When the engine is not startingup and in warmup operation, the routine jumps to step 302, where it isdetermined that the engine is operating under low load. When the engineis not operating under low load, the routine proceeds to step 303, wherethe exhaust control valve 24 is fully opened, then the routine proceedsto step 304, where the injection of the main fuel Qm is controlled. Atthis time, the auxiliary fuel Qa is not injected.

On the other hand, when it is judged at step 300 that the engine isstarting up and in warmup operation, the routine proceeds to step 301,where it is determined if a predetermined set time has elapsed afterengine startup. When the set time has not elapsed, the routine proceedsto step 305. On the other hand, when the set time has elapsed, theroutine proceeds to step 302. When it is determined at step 302 that theengine is operating under low load as well, the routine proceeds to step305. At step 305, the exhaust control valve 24 is substantially fullyclosed.

Next, at step 306, the amount of injection of main fuel Qm (X in FIG. 9and FIG. 10) predetermined in accordance with the operating state of theengine is calculated. Next, at step 307, it is determined if the backpressure P detected by the pressure sensor 80 is lower than a value(P₀−α) smaller than the target value P₀ by exactly a constant value α.When P<P₀−α, the routine proceeds to step 308, where a constant value kmis added to the correction value ΔQm with respect to the main fuel Qm.On the other hand, when P≧P₀−α, the routine proceeds to step 309, whereit is determined if the back pressure P is higher than a value (P₀+α)larger than the target value P₀ by exactly a constant value α. WhenP>P₀+α, the routine proceeds to step 310, where a constant value km issubtracted from the correction value ΔQm.

Next, at step 311, the value of Qm plus ΔQm is made the final amount ofinjection Qm₀ of the main fuel. That is, when P<P₀−α, the main fuel isincreased. When P>P₀+α, the main fuel is decreased. Due to this, theback pressure P is controlled so that P₀−α<P<P₀+α. Next, at step 312,the injection of the auxiliary fuel Qa is controlled.

FIG. 19 shows the routine for operational control when controlling theback pressure by controlling the auxiliary fuel Qa.

Referring to FIG. 19, first, at step 400, it is determined if the engineis starting up and in warmup operation. When the engine is starting upand in warmup operation, the routine jumps to step 402, where it isdetermined if the engine is operating under low load. When the engine isnot operating under low load, the routine proceeds to step 403, wherethe exhaust control valve 24 is fully opened, then the routine proceedsto step 404, where the injection of the main fuel Qm is controlled. Atthis time, the auxiliary fuel Qa is not injected.

On the other hand, when it is judged at step 400 that the engine isstarting up and in warmup operation, the routine proceeds to step 401,where it is determined if a predetermined set time has elapsed fromafter engine startup. When the set time has not yet elapsed, the routineproceeds to step 405. On the other hand, when the set time has elapsed,the routine proceeds to step 402. When it is determined at step 402 thatthe engine is operating under low load, the routine proceeds to step405. At step 405, the exhaust control valve 24 is substantially fullyclosed, then at step 406, the injection of the main fuel Qm iscontrolled. That is, if the engine is starting up and in warmupoperation, the amount of injection of the main fuel Qm is made the Xshown in FIG. 9, while when the engine is operating under low load, theamount of injection of the main fuel Qm is made the X shown in FIG. 10.

Next, at step 407, the amount of injection of auxiliary fuel Qapredetermined in accordance with the operating state of the engine iscalculated. Next, at step 408, it is determined if the back pressure Pdetected by the pressure sensor 80 is lower than a value (P₀−α) smallerthan the target value P₀ by exactly a constant value α. When P<P₀−α, theroutine proceeds to step 409, where a constant value ka is added to thecorrection value ΔQa for the auxiliary fuel Qa. On the other hand, whenP≧P₀−α, the routine proceeds to step 410, where it is determined if theback pressure P is higher than a value (P₀+α) larger than the targetvalue P₀ by exactly a constant value a. When P>P₀+α, the routineproceeds to step 411, where a constant value ka is subtracted from thecorrection value ΔQa.

Next, at step 412, the value of Qa plus ΔQa is made the final injectionQa₀ of auxiliary fuel. That is, when P<P₀−α, the auxiliary fuel isincreased, while when P>P₀+α, the auxiliary fuel is decreased. Due tothis, the back pressure is controlled so that P₀−α<P<P₀+α.

FIG. 20 shows a routine for operational control when controlling theback pressure by controlling the amount of intake air.

Referring to FIG. 20, first, at step 500, it is determined if the engineis starting up and in warmup operation. When the engine is starting upand in warmup operation, the routine jumps to step 502, where it isdetermined if the engine is operating under low load. When the engine isnot operating under low load, the routine proceeds to step 503, wherethe exhaust control valve 24 is fully opened, then the routine proceedsto step 504, where the injection of the main fuel Qm is controlled. Atthis time, the auxiliary fuel Qa is not injected.

On the other hand, when it is judged at step 500 that the engine isstarting up and in warmup operation, the routine proceeds to step 501,where it is determined if a predetermined set time has elapsed fromafter engine startup. When the set time has not yet elapsed, the routineproceeds to step 505. On the other hand, when the set time has elapsed,the routine proceeds to step 502. When it is determined at step 502 thatthe engine is operating under low load, the routine proceeds to step505. At step 505, the exhaust control valve 24 is substantially fullyclosed.

Next, at step 506, the target opening degree θ of the throttle valve 18predetermined in accordance with the operating state of the engine iscalculated. Next, at step 507, it is determined if the back pressure Pdetected by the pressure sensor 80 is lower than a value (P₀−α) smallerthan the target value P₀ by exactly a constant value α. When P<P₀−α, theroutine proceeds to step 508, where a constant value k is added to thecorrection value Δθ with respect to the target opening degree θ of thethrottle valve 18. On the other hand, when P≧P₀−α, the routine proceedsto step 509, where it is determined if the back pressure P is higherthan a value (P₀+α) greater than the target value P₀ by exactly aconstant value α. When P>P₀+α, the routine proceeds to step 510, where aconstant value k is subtracted from the correction value Δθ.

Next, a step 511, the value of θ plus Δθ is made the final targetopening degree θ₀ of the throttle valve 18. That is, when P<P₀−α, theopening degree of the throttle valve 18 is increased, so the amount ofintake air is increased, while when P>P₀+α, the opening degree of thethrottle valve 18 is decreased, so the amount of intake air isdecreased. Due to this, the back pressure P is controlled so thatP₀−α<P<P₀+α. Next, at step 512, the injection of the main fuel Qm iscontrolled. That is, when the engine is starting up and in warmupoperation, the amount of injection of main fuel Qm is made the X shownin FIG. 9, while when the engine is operating under low load, the amountof injection of the main fuel Qm is made the X shown in FIG. 10. Next,at step 513, the injection of the auxiliary fuel Qa is controlled.

Now, as explained above, if the exhaust control valve 24 issubstantially fully closed, the amount of injection X of the main fuelQm is increased, and the auxiliary fuel Qa is additionally injected, theamount of unburned hydrocarbons exhausted into the atmosphere can begreatly reduced without the torque generated by the engine falling. Whenthe required load of the engine becomes high during warmup operation,however, if the exhaust control valve 24 is held in a substantiallyfully closed state, the torque generated by the engine ends up fallingwith respect to the required value. Therefore, it is necessary to openthe exhaust control valve 24 when the required load of the enginebecomes high during warmup operation.

In this case, however, if the exhaust control valve 24 is fully opened,a fall in the torque generated by the engine is prevented, but theoxidation reaction of the unburned hydrocarbons in the exhaust passagedoes not proceed and therefore the amount of unburned hydrocarbonsexhausted into the atmosphere is increased. Therefore, fully opening theexhaust control valve 24 when the required load of the engine becomeshigh is not preferable. Therefore, in the embodiment shown in FIG. 21and FIG. 22, when a representative value representing the required loadof the engine becomes high, the opening degree of the exhaust controlvalve 24 is made larger along with the rise in the representative value.Due to this, the exhaust of the unburned hydrocarbons into theatmosphere can be suppressed while suppressing the fall in the torquegenerated by the engine.

In this embodiment, the amount of depression L of the accelerator pedal50 is used as a representative value representing the required torque.The relationship between the amount of depression L of the acceleratorpedal 50 and the opening degree of the exhaust control valve 24 in thiscase is shown in FIG. 21. As shown in FIG. 21, in this embodiment, whenthe amount of depression L of the accelerator pedal 50 is smaller than apredetermined first amount of depression Lm, the exhaust control valve24 is substantially fully closed; when the amount of depression L of theaccelerator pedal 50 becomes larger than a predetermined second amountof depression Ln (>Lm), the exhaust control valve 24 is fully opened;while when the amount of depression L of the accelerator pedal 50 isbetween the first amount of depression Lm and the second amount ofdepression Ln, the opening degree of the exhaust control valve 24 ismade larger along with an increase of the amount of depression L of theaccelerator pedal 50.

That is, between the first amount of depression Lm and the second amountof depression Ln, the opening degree of the exhaust control valve 24 isset to the smallest opening degree giving the highest back pressurewithout the torque generated by the engine falling much at all withrespect to the required generated torque. Therefore, when the amount ofdepression L of the accelerator pedal 50 is between the first amount ofdepression Lm and the second amount of depression Ln, if the openingdegree of the exhaust control valve 24 is made the opening degree shownin FIG. 21 corresponding to the amount of depression L of theaccelerator pedal 50, the torque generated by the engine does not fallmuch at all and the oxidation reaction of the unburned hydrocarbons inthe exhaust passage is promoted, so the amount of the unburnedhydrocarbons exhausted into the atmosphere can be reduced.

Note that as will be understood from FIG. 21, at the time of slowacceleration operation where the-amount of depression L of theaccelerator pedal 50 changes from L<Lm to Lm<L<Ln, the exhaust controlvalve 24 is opened up to the opening degree in accordance with theamount of depression L of the accelerator pedal 50, but at the time offast acceleration operation where the amount of depression L of theaccelerator pedal 50 changes from L<Lm to L>Ln, the exhaust controlvalve 24 is fully opened. Therefore, the opening degree of the exhaustcontrol valve 24 changes in accordance with the degree of acceleration.The higher the degree of acceleration, the greater the opening degree ofthe exhaust control valve 24.

On the other hand, the amount of drop of the generated torque withrespect to the required generated torque when the exhaust control valve24 is fully opened under the same engine operating state becomes smallerthe larger the opening degree of the exhaust control valve 24.Therefore, in this embodiment, between the first amount of depression Lmand the second amount of depression Ln, as shown in FIG. 21, theincrease in the amount of injection X of the main fuel Qm with respectto the optimum amount of injection Y of the main fuel Qm when theexhaust control valve 24 is fully opened under the same engine operatingconditions is reduced along with an increase in the amount of depressionL of the accelerator pedal 50.

Further, as shown in FIG. 21, the amount of injection of auxiliary fuelQa decreases the greater the amount of depression L of the acceleratorpedal 50. In the embodiment shown in FIG. 21, when L>Ln, the injectionof the auxiliary fuel Qa is stopped.

Further, even in this embodiment, as shown in FIG. 10, at the time ofengine low load operation, the exhaust control valve 24 is substantiallyfully closed, the amount of injection X of the main fuel Qm is increasedover the optimum amount of increase Y of the main fuel Qm when theexhaust control valve 24 is fully opened under the same engine operatingconditions, and the auxiliary fuel Qa is additionally injected. Next, ifthe engine is not in the low load operating state, the exhaust controlvalve 24 is immediately fully opened.

FIG. 22 shows the routine for operational control.

Referring to FIG. 22, first, at step 600, it is judged if the engine isstarting up and in warmup operation. If the engine is starting up and inwarmup operation, the routine proceeds to step 601, where it isdetermined if a predetermined set time has elapsed after the startup ofthe engine. When the set time has not elapsed, the routine proceeds tostep 602. On the other hand, when it is determined at step 600 that theengine is starting up and in warmup operation or when it is determinedat step 601 that the set time has elapsed, the routine proceeds to step605, where it is determined if the engine load is lower than a set load,that is, the engine is operating under low load. At the time of low loadoperation, the routine proceeds to step 602.

At step 602, the opening degree of the exhaust control valve 24 iscontrolled. That is, at the time of engine startup and warmup operation,the opening degree of the exhaust control valve 24 is made an openingdegree in accordance with the amount of depression L of the acceleratorpedal 50 shown in FIG. 21. As opposed to this, when it is judged at step605 that the engine is operating under low load, the exhaust controlvalve 24 is substantially fully closed. Next, at step 603, the injectionof the main fuel Qm is controlled. That is, if the engine is starting upand in warmup operation, the amount of injection of the main fuel Qm ismade the X shown in FIG. 21. When it is judged at step 605 that theengine is operating under low load, the amount of injection of the mainfuel Qm is made the X shown in FIG. 10. Next, at step 604, the injectionof the auxiliary fuel Qa is controlled.

On the other hand, when it is judged at step 605 that the engine is notoperating under low load, the routine proceeds to step 606, where. theexhaust control valve 24 is fully opened, then the routine proceeds tostep 607, where the injection of the main fuel Qm is controlled. At thistime, the auxiliary fuel Qa is not injected.

In the embodiments explained up to here, however, just when the enginewas starting up, the exhaust control valve 24 was substantially fullyclosed, the main fuel Qm was increased, and the auxiliary fuel Qa wasadditionally injected. At the time of engine startup, however, thetemperature of the engine is low, so if the auxiliary fuel is injectedat this time, the auxiliary fuel will not sufficiently burn andtherefore there will be the danger of the amount of unburnedhydrocarbons generated conversely increasing. Therefore, in theembodiment explained below, the injection of the auxiliary fuel at thetime of engine startup is controlled so that a large amount of unburnedhydrocarbons is not generated at the time of engine startup.

FIG. 23 is an overview of an internal combustion engine used at thistime. As will be understood from FIG. 23, in this internal combustionengine, the operating signal of the ignition switch 53 and the operatingsignal of the starter switch 54 are input to the input port 45.

Next, an explanation will be given, referring to FIG. 24, of anembodiment where the amount of injection of the auxiliary fuel isgradually increased after the engine starts operating under its ownpower at the time of engine startup so as to prevent the generation of alarge amount of unburned hydrocarbons at the time of engine startup.Note that FIG. 24 shows the operation of the ignition switch 53, thechange in the opening degree of the exhaust control valve 24, theoperation of the starter switch 54, the engine speed N, the change inthe amount of injection Qm of the main fuel, and the amount of injectionQa of the auxiliary fuel.

As shown in FIG. 24, while the ignition switch 53 is off, the exhaustcontrol valve 24 is held in the fully open state. When the ignitionswitch 53 is switched from off to on, the exhaust control valve 24 isswitched from the fully opened state to the substantially fully closedstate. Next, when the starter switch 54 is turned on, the injection ofthe main fuel Qm is started. The change in the amount of injection Qm ofthe main fuel at this time is shown by the solid line X in FIG. 24.

That is, the solid line X in FIG. 24 shows the optimum amount ofinjection of the main fuel Qm when substantially fully closing theexhaust control valve 24, while the broken line X₀ shows the optimumamount of injection of the main fuel Qm when fully opening the exhaustcontrol valve 24. Therefore, in this embodiment as well, it is learnedthat at the time of engine startup and warmup operation, the amount ofinjection X of the main fuel Qm is increased from even the optimumamount of injection X₀ of the main fuel Qm when the exhaust controlvalve 24 is fully opened under the same engine operating conditions.

While the engine is being driven by the starter motor, the engine speedN is maintained at a substantially constant speed of about 200 rpm. Whenthe engine starts operating under its own power, the engine. speed Nrapidly rises. In this case, in this embodiment, when the engine speed Nexceeds a predetermined speed, for example, 400 rpm, it is judged thatthe engine has started operating under its own power. When it is judgedthat the engine has started to operate under its own power, the amountof injection X of the main fuel Qm is rapidly decreased.

On the other hand, in FIG. 24, the broken line Y₀ shows the targetamount of injection of the auxiliary fuel Qa predetermined in accordancewith the operating state of the engine. The target amount of injectionY₀ shows the amount of injection of fuel required for maintaining thetemperature of the exhaust gas at the exhaust port 11 outlet at thetarget temperature, for example, 800° C. The target amount of injectionY₀ increases along with a decrease of the amount of injection X of themain fuel. The target amount of injection Y₀ of the auxiliary fuel Qa isstored in the ROM 42 in advance as a function of the required load L andthe engine speed N.

In FIG. 24, the solid line Y shows an actual amount of injection of theauxiliary fuel Qa. As shown in FIG. 24, in this embodiment, if it isjudged that the engine has started operating under its own power, theinjection of the auxiliary fuel Qa is started, then the amount ofinjection Y of the auxiliary fuel Qa is gradually increased toward thetarget amount of injection Y₀.

Right after the engine starts operating under its own power, thetemperature of the engine body 1 is low. Therefore, at this time, if alarge amount of auxiliary fuel Qa is injected, not all of the injectedfuel is burned well, so a large amount of unburned hydrocarbons isgenerated. Therefore, at this time, a small amount of auxiliary fuel Qais injected. On the other hand, after the engine starts operating underits own power, the temperature of the engine body 1 gradually rises andtherefore even if the amount of injection of the auxiliary fuel Qa isincreased, the auxiliary fuel Qa burns well. Therefore, after the enginestarts operating under its own power, as shown in FIG. 24, the amount ofinjection Y of the auxiliary fuel Qa is gradually increased toward thetarget amount of injection Y₀.

FIG. 25 shows a routine for operational control.

Referring to FIG. 25, first, at step 700, it is determined if theignition switch 53 has been switched from off to on. When the ignitionswitch 53 is switched from off to on, the routine proceeds to step 701,where the exhaust control valve 24 is switched from the fully openedstate to the substantially fully closed state. Next, at step 702, it isdetermined if a predetermined set time has elapsed from when the enginestarts operating, for example, from when the engine starts operating onits own power.

When the set time has not elapsed, the routine proceeds to step 703,where the injection of the main fuel Qm is controlled. That is, theamount of injection of the main fuel Qm is made the X shown in FIG. 24.Next, at step 704, the injection of the auxiliary fuel Qa is injected.That is, the amount of injection of the auxiliary fuel Qa is made the Yshown in FIG. 24. On the other hand, when it is judged at step 702 thatthe set time has elapsed, the routine proceeds to step 705, where theexhaust control valve 24 is fully opened, then the routine proceeds tostep 706, where the injection of the main fuel Qm is controlled. At thistime, the auxiliary fuel Qa is not injected.

FIG. 26 shows the control of the injection of the auxiliary fuelperformed at step 704 of FIG. 25 for working the embodiment shown inFIG. 24.

Referring to FIG. 26, first, at step 800, it is judged if the enginespeed N has become higher than 400 rpm, that is, if the engine hasstarted to operate on its own power. When N≦400 rpm, the routineproceeds to step 804, where the amount of injection Qa of the auxiliaryfuel is made zero. That is, the injection of the auxiliary fuel isstopped. As opposed to this, when N>400 rpm, the routine proceeds tostep 801, where a constant value ΔQ is added to the amount of injectionQa of the auxiliary fuel. Next, at step 802, it is judged if the amountof injection Qa of the auxiliary fuel has become larger than the targetamount of injection XQa in accordance with the operating state of theengine shown by Y₀ at FIG. 24. When Qa>XQa, the routine proceeds to step803, where Qa is made XQa. Therefore, if the engine starts operatingunder its own power, the amount of injection Qa of the auxiliary fuel isgradually increased toward the target amount of injection XQa. When theamount of injection Qa of the auxiliary fuel reaches the target amountof injection XQa, the amount of injection Qa of the auxiliary fuel isthen maintained at the target amount of injection XQa.

FIG. 27 shows another embodiment. In this embodiment, as shown by thesolid line Y in FIG. 27, the amount of injection Qa of auxiliary fuel isgradually increased from before the engine starts operating under itsown power, that is, from when the starter switch 54 is switched from offto on. The amount of injection Qa. of the auxiliary fuel reaches thetarget amount of injection Y₀ after the engine starts operating underits own power.

FIG. 28 shows the control of the injection of auxiliary fuel performedat step 704 in FIG. 25 for working the embodiment shown in FIG. 27.

Referring to FIG. 28, first, at step 900, it is determined if thestarter switch 54 has been switched from off to on. When the starterswitch 54 has been switched from off to on, the routine proceeds to step901, where the starter flag is set, then the routine proceeds to step902.

At step 902, it is determined if the starter flag has been set. When thestarter flag has not been set, that is, when the engine is stopped, theroutine proceeds to step 906, where the amount of injection Qa of theauxiliary fuel is made zero. That is, the injection of auxiliary fuel isstopped. As opposed to this, when the starter flag is set, the routineproceeds to step 903, where the constant value ΔQ is added to the amountof injection Qa of the auxiliary fuel. Next, at step 904, it isdetermined if the amount of injection Qa of the auxiliary fuel hasbecome larger than the target amount of injection XQa in accordance withthe operating state of the engine shown by Y₀ in FIG. 27. When Qa>XQa,the routine proceeds to step 905, where Qa is made XQa. Therefore, whenthe starter switch 54 is switched from off to on, the amount ofinjection Qa of the auxiliary fuel is gradually increased toward thetarget amount of injection XQa. When the amount of injection Qa of theauxiliary fuel reaches the target amount of injection XQa, the amount ofinjection Qa of the auxiliary fuel then is maintained at the targetamount of injection XQa.

FIG. 29 shows a further embodiment. In this embodiment, as shown by thesolid line Y in FIG. 29, the injection of the auxiliary fuel Qa isstarted by the target amount of injection Y₀ after a predetermined timeelapses from when the engine starts to operate under its own power. Thatis, in this embodiment, even if the auxiliary fuel Qa is injected by thetarget amount of injection Y₀, the auxiliary fuel Qa starts to beinjected at the timing when all of the fuel can be burned well.

FIG. 30 shows the control of the injection of auxiliary fuel performedat step 704 in FIG. 25 for working the embodiment shown in FIG. 29.Referring to FIG. 30, first, at step 1000, it is determined if theengine speed N has become higher than 400 rpm, that is, if the enginehas started operating under its own power. When N>400 rpm, the routineproceeds to step 1001, where it is determined if a constant time haselapsed from when N>400 rpm. When it is determined at step 1000 thatN≦400 rpm or it is determined at step 1001 that a constant time has notelapsed after N>400 rpm, the routine proceeds to step 1005, where theamount of injection Qa of auxiliary fuel is made zero. That is, theinjection of the auxiliary fuel is stopped.

As opposed to this, when it is judged at step 1101 that a constant timehas not elapsed from when N>400 rpm, the routine proceeds to step 1002,where the constant value ΔQ is added to the amount of injection Qa ofthe auxiliary fuel. Next, at step 1003, it is judged if the amount ofinjection Qa of the auxiliary fuel has become larger than the targetamount of injection XQa in accordance with the operating state of theengine shown by Y₀ in FIG. 29. When Qa>XQa, the routine proceeds to step1003, where Qa is made XQa. Therefore, when a constant time has elapsedfrom when the engine starts operating under its own power, the amount ofinjection Qa of the auxiliary fuel is gradually increased up to thetarget amount of injection XQa, then the amount of injection Qa of theauxiliary fuel is maintained at the target amount of injection XQa. Inthis case, if ΔQ=XQa is set, as shown in FIG. 29, when a constant timehas elapsed from when the engine starts operating under its own power,the amount of injection Qa of auxiliary fuel is increased all at once upto the target amount of injection XQa, then the amount of injection Qaof the auxiliary fuel is maintained at the target amount of injectionXQa.

Next, an explanation will be made of an embodiment where the oxidationreaction of the unburned hydrocarbons in the exhaust passage ispromoted.

In the embodiment shown in FIG. 31, the exhaust ports of the cylinders#1, #2, #3, and #4 are connected to the corresponding tubes 90 a of theexhaust manifold 90. Inside each tube 90 a is formed an expanded volumechamber 91 having a far larger sectional area than the sectional area ofthe respective exhaust ports. If expanded volume chambers 91 are formedin the tubes 90 a of the exhaust manifold in this way, the flow rate ofthe exhaust gas becomes slower in the expanded volume chambers 91 andtherefore the exhaust gas exhausted from the exhaust ports remains inthe exhaust passage upstream of the exhaust control valve 24 under ahigh temperature over a long period. If the exhaust gas remains in theexhaust passage upstream of the exhaust control valve 24 under a hightemperature over a long period, the oxidation reaction of the unburnedhydrocarbons in the exhaust passage is promoted and therefore the amountof unburned hydrocarbons exhausted into the atmosphere is furtherreduced.

In this case, the longer the time the exhaust gas remains in the exhaustpassage upstream of the exhaust control valve 24, the greater the amountof reduction of the unburned hydrocarbons. Further, the larger thevolume of the expanded volume chamber 91, the longer the time itremains. In the embodiment shown in FIG. 31, to make the time ofresidence of the exhaust gas longer, the sectional area of the expandedvolume chamber 91 is made at least two times the sectional area of theexhaust port and the axial direction length of the expanded volumechamber 91 is made substantially the same as the diameter of theexpanded volume chamber 91 or at least the diameter of the expandedvolume chamber 91.

As explained above, if expanded volume chambers 91 are provided in thetubes 90 a of the exhaust manifold 90, the oxidation reaction of theunburned hydrocarbons in the exhaust gas is promoted. Therefore, theseexpanded volume chambers 91 form the oxidation reaction promoting meansfor the unburned hydrocarbons. FIG. 32 shows another example of thisoxidation reaction promoting means. In the example shown in FIG. 32, anexpanded volume chamber 92 connected to the exhaust ports of thecylinders and common for all cylinders is provided adjoining the outletsof the exhaust ports. In this example, the flow rate of the exhaust gasbecomes slower in the expanded volume chamber 92 and therefore theoxidation reaction of the unburned hydrocarbons is promoted.

On the other hand, it is possible to promote the oxidation reaction ofthe unburned hydrocarbons in the exhaust gas by warming the exhaust gasas well. FIG. 33 and FIG. 34 show an example of an oxidation reactionpromoting means for promoting the oxidation reaction of unburnedhydrocarbons by warming the exhaust gas.

Referring to FIG. 33 and FIG. 34, a double wall structure exhaustmanifold or reactor 93 is provided in the exhaust passage. The exhaustmanifold or reactor 93 is connected to the exhaust port 11 of eachcylinder through a tube 94 of the double wall structure. That is, theexhaust manifold or reactor 93 is comprised of a liner 93 b and an outerframe 93 a surrounding the liner 93 b through a space from the liner 93b. The tube 94 is comprised of a linear 94 b and an outer frame 94 asurrounding the liner 94 b through a space from the liner 94 b. As shownin FIG. 34, the liner 94 b extends up to the inside of the correspondingexhaust port 11. A space is formed around the liner 94 b in the exhaustport 11 as well. That is, the inside of the exhaust port 11 is alsogiven a double wall structure.

Further, as shown in FIG. 33, the exhaust pipe 21, catalytic converter70 a, and exhaust pipe 21 a all have double wall structures. Therefore,the exhaust gas exhausted from the combustion chamber 5 is held at ahigh temperature by the adiabatic action due to the double wallstructure. Therefore, when the exhaust control valve 24 is substantiallyfully closed, the oxidation action of the unburned hydrocarbons in theexhaust gas is greatly promoted. Further, in the example shown in FIG.33 and FIG. 34, the exhaust manifold or reactor 93 form an expandedvolume chamber and therefore the oxidation reaction of the unburnedhydrocarbons is further promoted.

As another oxidation reaction promoting means for promoting theoxidation reaction of unburned hydrocarbons by warming the exhaust gas,there is also the method of forming the exhaust manifold or exhaust pipefrom a material with a low heat conductivity or surrounding the exhaustmanifold or exhaust pipe by an insulating material.

FIG. 35 shows still another embodiment of an oxidation reactionpromoting means. In this embodiment, as shown in FIG. 35, in the regionI where the temperature TE of the exhaust gas rises toward thedownstream side, the sectional area of the flow channel of the exhaustgas gradually is increased toward the downstream side, while in theregion II where the temperature TE of the exhaust gas falls toward thedownstream side, the sectional area of the flow channel of the exhaustgas is gradually reduced toward the downstream side. Specificallyspeaking, in the region I, the sectional area of the flow channel of theexhaust port 11 and the sectional area of the flow channel of the tube96 of the exhaust manifold 95 are gradually increased toward thedownstream side, while in the region II, the sectional area of the flowchannel of the tube 96 of the exhaust manifold 95 is gradually reducedtoward the downstream side.

That is, as explained above, when the exhaust control valve 24 issubstantially fully closed and auxiliary fuel Qa is injected, theunburned hydrocarbons in the exhaust gas exhausted from the combustionchamber 5 is gradually oxidized while flowing toward the downstreamside. As a result, the temperature TE of the exhaust gas exhausted fromthe combustion chamber 5 gradually rises the further downstream due tothe heat of the oxidation reaction of the unburned hydrocarbons as shownin FIG. 35. Next, when going further downstream, the temperature TE ofthe exhaust gas gradually falls due to the cooling action of the outsideair. That is, in the region I where the temperature TE of the exhaustgas rises, the oxidation reaction of the unburned hydrocarbons becomesactive, while in the region II, the oxidation reaction of the unburnedhydrocarbons does not become that active.

In this case, to promote the oxidation reaction of the unburnedhydrocarbons, it is effective to further activate the oxidation reactionof the unburned hydrocarbons in the region I. To activate the oxidationreaction, it is sufficient to make the time the exhaust gas remainsunder a high temperature longer. For that, it is sufficient to increasethe sectional area of the flow channel of the exhaust gas. Therefore, inthe region I, the sectional area of the flow channel is graduallyincreased toward the downstream side. Note that if the sectional area ofthe flow channel is gradually increased toward the downstream side, theflow of exhaust gas peels off from the inner walls of the exhaust port11 and exhaust manifold tubes 96, so the cooling action on the exhaustgas becomes weaker and therefore it is possible to promote the oxidationreaction of the unburned hydrocarbons more.

On the other hand, in the region II, the oxidation reaction of theunburned hydrocarbons is originally not that active. Therefore, even ifpromoting the oxidation reaction of unburned hydrocarbons in the regionII, a large effect of reduction of the unburned hydrocarbons cannot beobtained. Further, if the sectional area of the flow channel of theexhaust gas in the region II is increased toward the downstream side,there is the problem that the dimensions of the exhaust system end upbecoming extremely large. Further, there is the problem that the outputof the engine falls since the exhaust pulsation dies down. Therefore, inthe region II, the area of the flow channel of the exhaust gas isgradually reduced toward the downstream side.

Note that in the embodiment shown from FIG. 31 to FIG. 35, it is alsopossible to arrange a catalyst having an oxidation function in theexhaust ports, in the exhaust manifold, or in the exhaust manifold tubesso as to further promote the oxidation reaction of the unburnedhydrocarbons.

LIST OF REFERENCE NUMERALS

5 . . . combustion chamber

6 . . . fuel injector

7 . . . spark plugs

11 . . . exhaust port

13 . . . surge tank

18 . . . throttle valve

19, 20, 90 . . . exhaust manifold

21, 21 a, 22 . . . exhaust pipe

24 . . . exhaust control valve

70, 71, 72 . . . catalyst

What is claimed is:
 1. An exhaust gas purification device of an internalcombustion engine wherein an exhaust control valve is arranged apredetermined distance away from an outlet of an engine exhaust portinside an exhaust passage connected to the outlet of the exhaust port;when it is judged that the amount of unburned hydrocarbons exhaustedinto the atmosphere is to be reduced, the exhaust control valve issubstantially fully closed and, in addition to burning the main fuelinjected into the combustion chamber under excess air to generate engineoutput, auxiliary fuel is additionally injected into the combustionchamber at a predetermined timing in the expansion stroke or exhauststroke where the auxiliary fuel can be burned so that the amount ofunburned hydrocarbons produced in the combustion chamber is reduced andthe oxidizing reaction of hydrocarbons in the exhaust port and theexhaust passage upstream of the exhaust control valve is promoted; andwhen the exhaust control valve is substantially fully closed, the amountof injection of main fuel is increased compared with the case where theexhaust control valve is fully opened under the same engine operatingconditions.
 2. An exhaust gas purification device of an internalcombustion engine as set forth in claim 1, wherein it is judged that theamount of exhaust of unburned hydrocarbons into the atmosphere should bereduced when the engine is in warmup operation.
 3. An exhaust gaspurification device of an internal combustion engine as set forth inclaim 1, wherein it is judged that the amount of exhaust of unburnedhydrocarbons into the atmosphere should be reduced when the engine isoperating under low load.
 4. An exhaust gas purification device of aninternal combustion engine as set forth in claim 1, wherein the amountof injection of auxiliary fuel is reduced along with an increase of theamount of injection of main fuel when it is judged that the amount ofexhaust of unburned hydrocarbons into the atmosphere should be reduced.5. An exhaust gas purification device of an internal combustion engineas set forth in claim 1, wherein auxiliary fuel in addition to main fuelis burned under excess air.
 6. An exhaust gas purification device of aninternal combustion engine as set forth in claim 1, wherein an air-fuelmixture formed in a limited region in the combustion chamber by the mainfuel is ignited by a spark plug and then the auxiliary fuel isadditionally injected.
 7. An exhaust gas purification device of aninternal combustion engine as set forth in claim 1, wherein a catalystis arranged in the exhaust passage.
 8. An exhaust gas purificationdevice of an internal combustion engine as set forth in claim 7, whereinthe catalyst is comprised of an oxidation catalyst, three-way catalyst,NO_(x) absorbent, or hydrocarbon absorbing catalyst.
 9. An exhaust gaspurification device of an internal combustion engine as set forth inclaim 7, wherein judging means is provided for judging if the catalystis higher in temperature than an activation temperature and it is judgedthat the amount of exhaust of unburned hydrocarbons into the atmosphereshould be reduced when the catalyst is lower in temperature than theactivation temperature and the engine is in warmup operation.
 10. Anexhaust gas purification device of an internal combustion engine as setforth in claim 7, wherein judging means is provided for judging if thecatalyst is higher in temperature than an activation temperature and itis judged that the amount of exhaust of unburned hydrocarbons into theatmosphere should be reduced when the catalyst is lower in temperaturethan the activation temperature and the engine is operating under lowload.
 11. An exhaust gas purification device of an internal combustionengine as set forth in claim 7, wherein the catalyst is arranged in theexhaust passage upstream of the exhaust control valve.
 12. An exhaustgas purification device of an internal combustion engine as set forth inclaim 1, wherein when the amount of exhaust of unburned hydrocarbonsinto the atmosphere should be reduced, the combustion in the combustionchamber is controlled so that one of the pressure or temperature of theexhaust gas in the exhaust passage upstream of the exhaust control valvebecomes a target value.
 13. An exhaust gas purification device of aninternal combustion engine as set forth in claim 12, wherein thecombustion in the combustion chamber is controlled by controlling atleast one of an amount of injection of main fuel, amount of injection ofauxiliary fuel, and amount of intake air.
 14. An exhaust gaspurification device of an internal combustion engine as set forth inclaim 13, wherein at least one of the amount of injection of main fuel,amount of injection of auxiliary fuel, and amount of intake air isincreased when any one of the pressure or temperature of the exhaust gasin the exhaust passage upstream of the exhaust control valve is lowerthan the target value.
 15. An exhaust gas purification device of aninternal combustion engine as set forth in claim 1, wherein the exhaustcontrol valve is switched from the fully open state to the substantiallyfully closed state at the time of engine startup.
 16. An exhaust gaspurification device of an internal combustion engine as set forth inclaim 15, wherein a vacuum tank accumulating vacuum and a vacuumoperated type actuator for driving the exhaust control valve areprovided, and the actuator is operated by vacuum accumulated in thevacuum tank.
 17. An exhaust gas purification device of an internalcombustion engine as set forth in claim 1, wherein the exhaust controlvalve is substantially fully closed when a representative valuerepresenting the required load is lower than a predetermined value untila predetermined time elapses after engine startup and wherein theopening degree of the exhaust control valve is made larger along with anincrease in the representative value when the representative valuebecomes larger than a predetermined value until a predetermined timeelapses after engine startup.
 18. An exhaust gas purification device ofan internal combustion engine as set forth in claim 17, wherein theincrease in the amount of injection of main fuel is reduced along withan increase in the opening degree of the exhaust control valve until apredetermined time after engine startup elapses.
 19. An exhaust gaspurification device of an internal combustion engine as set forth inclaim 17, wherein the increase in the amount of injection of auxiliaryfuel is reduced along with an increase in the opening degree of theexhaust control valve until a predetermined time after engine startupelapses.
 20. An exhaust gas purification device of an internalcombustion engine as set forth in claim 17, wherein the exhaust controlvalve is substantially fully closed when the required load is lower thana set load after a predetermined time elapses after engine startup andwherein the exhaust control valve is fully opened when the required loadbecomes higher than even the set load after a predetermined time elapsesafter engine startup.
 21. An exhaust gas purification device of aninternal combustion engine as set forth in claim 20, wherein theoxidation reaction promoting means is comprised of an exhaust port orexhaust passage gradually increased in sectional area of flow channeltoward the downstream side, then gradually reduced in sectional area offlow channel toward the downstream side.
 22. An exhaust gas purificationdevice of an internal combustion engine as set forth in claim 1, whereinwhen starting engine operation, after the engine starts operating underits own power, the amount of injection of auxiliary fuel is made thetarget amount of injection predetermined in accordance with theoperating state of the engine.
 23. An exhaust gas purification device ofan internal combustion engine as set forth in claim 22, wherein theamount of injection of the auxiliary fuel is increased at one time up tothe target amount of injection when making the amount of injection ofthe auxiliary fuel the target amount of injection.
 24. An exhaust gaspurification device of an internal combustion engine as set forth inclaim 22, wherein the amount of injection of the auxiliary fuel isgradually increased toward the target amount of injection when makingthe amount of injection of the auxiliary fuel the target amount ofinjection.
 25. An exhaust gas purification device of an internalcombustion engine as set forth in claim 24, wherein the amount ofinjection of the auxiliary fuel is gradually increased toward the targetamount of injection after the engine starts operating on its own power.26. An exhaust gas purification device of an internal combustion engineas set forth in claim 24, wherein the amount of injection of theauxiliary fuel is gradually increased toward the target amount ofinjection before the engine starts operating on its own power.
 27. Anexhaust gas purification device of an internal combustion engine as setforth in claim 1, wherein oxidation reaction promoting means forpromoting the oxidation reaction of unburned hydrocarbons in the exhaustgas is provided in the engine exhaust port or at least the upstream partof the exhaust passage.
 28. An exhaust gas purification device of aninternal combustion engine as set forth in claim 27, wherein a catalystis arranged in one of the engine exhaust port or the exhaust passageupstream of the exhaust control valve.
 29. An exhaust gas purificationdevice of an internal combustion engine as set forth in claim 27,wherein the oxidation reaction promoting means promotes the oxidationreaction of the unburned hydrocarbons in the exhaust gas by reducing theflow rate of the exhaust gas.
 30. An exhaust gas purification device ofan internal combustion engine as set forth in claim 29, wherein theoxidation reaction promoting means is comprised of an expanded volumechamber provided in the exhaust passage.
 31. An exhaust gas purificationdevice of an internal combustion engine as set forth in claim 27,wherein the oxidation reaction promoting means promotes the oxidationreaction of the unburned hydrocarbons in the exhaust gas by keeping theexhaust gas warm.
 32. An exhaust gas purification device of an internalcombustion engine as set forth in claim 31, wherein the oxidationreaction promoting means is comprised of a double wall peripheral wallstructure.
 33. An exhaust gas purification device of an internalcombustion engine as set forth in claim 32, wherein an expanded volumechamber is provided inside the exhaust passage upstream of the exhaustcontrol valve and the distance from the inside of the export port to theinside of the expanded volume chamber is made a double wall structure.